1. Field of the Invention
The present invention relates to a gain control technique of a variable gain amplifier.
2. Description of Related Art
In an audiovisual apparatus, an audio signal amplifier is used in order to amplify an audio signal that is a weak electric signal and to supply the amplified signal to a sound output device such as a speaker or an earphone. In such an audio signal amplifier, a gain is changed to adjust a volume. For such adjustment of sound, for example, a variable gain amplifier described in Japanese Patent Application Laid-Open No. 2004-336129 is used.
In an audio signal amplifier, when a signal amplitude becomes discontinuous at the time of changing a gain, an unpleasant noise such as a click sound or the like is emitted from a speaker, so as to stimulate an auditory sense of a person listening to an audio.
In order to reduce such a click sound, it is necessary to modestly change the gain of the audio signal amplifier. A time constant circuit including a capacitor is used for gradually changing the gain of the audio signal amplifier. This time constant circuit produces a voltage that increases or decreases based on the charge/discharge of the capacitor. Based upon this voltage, the gain of the audio signal amplifier is controlled.
Here, a gain control of the audio signal amplifier is considered. A microcomputer is provided in electronic equipment with the audio signal amplifier provided therein. This microcomputer controls the gain of the audio signal amplifier. Here, the case of switching the gain to a large degree in the conventional audio signal amplifier is considered. For example, when a gain of an audio signal amplifier capable of switching a gain in 50 steps is to be switched from the largest gain to the smallest gain, switching the gain all at once causes distortion of an audio signal. Therefore, a control of switching the gain in a stepwise manner is performed by the microcomputer to prevent distortion of the audio signal. However, the problem of an increasing load of the microcomputer occurs when the gain of the audio signal amplifier is under software control by the microcomputer.